Question

You drop a 2.00 kg book to a friend who stands on the ground at distance D = 10.0 m below. If your friend's outstretched hands are at distance d = 1.50 m above the ground (Fig. 8-28), (a) how much work Wg does the gravitational force do on the book as it drops to her hands? (b) What is the change ΔU in the gravitational potential energy of the book–Earth system during the drop? If the gravitational potential energy U of that system is taken to be zero at ground level, what is U(c) when the book is released and (d) when it reaches her hands? Now take U to be 100 J at ground level and again find (e) Wg, (f) ΔU, (g) U at the release point, and (h) U at her hands.
The aswers are as follows:
a) 167J, b) -167J, c) 196J, d)29J, e)167J, f)-167J g)296J, and h) 129J

Answer 1

If your friend's outstretched hands are at distance d = 1.50 m above the ground, (a) work Wg does the gravitational force do on the book as it drops to her hands 167J, b) -167J, c) 196J, d)29J, e)167J, f)-167J g)296J, and h) 129J

a)  Work = mg(Ac)

              = 2 × 9.8 × (10 - 1.5) = 167j

b) ΔU = Ubc - Uab = Mg(bc) - Mg(ab)

          = mg(bc - ab)

           = 2 × 9.8 (1.5 - 10) = -167j

c) Potential energy at reach point v = mg(AB) = 2 × 9.8 × 10 = 196J

d) Potential energy at reach point U = mg (bc) = 2 × 9.8 × 1.5 = 29.4j

e) Total work wg = mg (ac) = 2 × 9.8 × 3.5 = 167j

f) ΔU = -wg = - 167j

g) ui = mg × AB + 100 = 196 + 100 = 296j

h) uf = mg × bc + 100 = 29.4 + 100 = 129j

Given the possibility of motion, gravitational forces can sort phases of various densities. Although high temperatures are not anticipated for the early Earth, widespread melting is one possibility. Sorting in response to the outgassing of deeply buried ices is a more feasible scenario. Such processes would have altered the moment of inertia, necessitating an adjustment to the angular velocity. Friction opposes gravitational settling and prevents it from being conservative, which makes quantification dubious. The ensuing phase separation and angular momentum alterations would not need to be as extensive as commonly believed if an iron protocore existed.

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